The "rosy" region of chromosome 3 of Drosophila melanogaster controls the biosynthesis of the enzyme xanthine dehydrogenase. This proposal requests support for the continuation of an investigation designed to dissect the component parts of this genetic unit both in terms of structure and function. Experiments, thus far completed, have provided a reasonably good outline of one of the components, that section of the region which provides the coded information for the amino acid sequence of the enzyme (structural element). Experiments proposed are designed to: 1. Precisely determine the boundaries of the structural element through a collaborative investigation involving sequencing and mapping XDH structural variants. 2. Mutagenesis experiments designed to produce variants which affect rate of synthesis, timing and tissue distribution of the enzyme (control element variants). 3. Mapping of such mutants relative to the structural element and to each other. 4. Developmental studies to uncover nature of the control system. This study is pertinent to an understanding of the regulation of gene action in higher organisms, a poorly understood area which must be illuminated if we are to understand both normal and abnormal development.